Idea: It is not always true that the work done by an external force is
stored as some form of potential energy. This is only true if the force is
conservative:

Wc = Fcext(xB - xA) = PEA - PEB.

(5)

Definition: The work a conservative force does on an object in
moving it from
A to B is path independent - it depends only on the end points of the motion.
Examples: the force of gravity and the spring force are conservative forces.
For a non-conservative (or dissipative) force, the work done in
going from A to B depends on the path taken. Examples: friction and
air resistance.